Thermal flowmeter



Jan. 10, 1956 J. H. LAUB THERMAL FLOWMETER 4 Sheets-Sheet 2 OriginalFiled May 21, 1946 IN V EN TOR (/OH/V HA/QR Y1 A08 ATTORNEY Jan. 10,1956 J. H. LAUB 2,729,976

THERMAL FLOWMETER Ongmal Flled May 21, 1946 4 sheets sheet 3 INVENTOR.domv HARRYZAUB BY {444 M A TTOAMEX Jan. 10, 1956 J. H. LAUB 2,729,976

THERMAL FLOWMETER Ongxnal Flled May 21, 1946 4 Sheets sheet 4 INVENTOR.d HIV f/A/P/PYZAl/B A TTORNEX United States Patent THERMAL FLOWMETERJohn Harry Laub, Short Hills, N. J.

Original application May 21, 1946, Serial No. 671,17h. Divided and thisapplication January 6, 1953, Serial No. 329,899

9 Claims. (Cl. 73-204) This invention deals with flow meters and isconcerned in particular with devices for measuring the rate of flow orquantity of flow of a liquid or other medium flowing througha conduit.

Flow meters as heretofore known have been based on mechanical principlessuch as the measurement of the displacement of an element in which theconfined flowing fluid causes such element (e. g. a nutating piston,propeller or vane) to rotate, the number of revolutions being a measureof the quanity of medium flowing through the conduit or pipe line, or onpressure drop occurring in the confined flowing medium passing through aVenturi tube or through an orifice, the pressure drop being a measure ofthe rate of flow.

The devices of the prior art suifer, however, from a number ofshortcomings. The mechanical means injected into the flowing mediumafiect the free flow thereof and increase the pressure drop across themeter. Great care must be exercised in their construction, often withoutsuccess, in order to avoid leakage of the medium, e. g. gasoline, ether,chlorine, etc., which may be inflamma ble, corrosive or otherwiseobjectionable, and such avoidance of leakage is particularly diflicultin meters of the volumetric displacement type where the rotary movementof the displacement element must be transmitted through the housing ofthe meter by mechanically moving parts. Furthermore, such devices arenot always accurate, especially where the flow rate may vary from timeto time from a low minimum to a high maximum; Such flow meters are ofthe direct indicating type and require, where remote indication isdesired, special auxiliary devices, e. g. magnetic plungers, electrictachometers, etc., for remote indication. The weight of such meters andthe relatively large space requiredfor their installation constitutefurther shortcomings where, as for instance in airplanes, small weightand small space occupancy are of prime importance. Pressure drop typemeters furthermore have a limited range and require special devices, e.g. devices for conversion of the square-root scale characteristic ofsuch meters into a linear scale characteristic. Also flow meters asheretofore known are usually designed for only one purpose and arenormally not capable of measuring the rate of flow as well as the totalquantity of flow.

There are also known to exist flowmeters operating on the electrocaloricprinciple or of the Thomas type in which the flow is measured by thedetermination of the effect of a known amount of heat dissipated into aflowing medium. However, the prior known instruments operating on thisprinciple have proven to be commercially unacceptable especially in themeasurement of the flow of liquids in that considerable errors inmeasurement due to fluctuations in fluid temperature, thermal time lags,etc. have not been overcome.

It is one object of this invention to provide a flowmeter operating onthe electrocaloric principle which practically removes errors inmeasurement due to thermal time lags, fluctuations in fluidtemperatures, etc. It is another object of this invention to provide anelectrocaloric 2,729,976 Patented Jan. 10, 1956 type flow meter whichshall be vibration and leak proof and incapable of interfering with thefree flow of the confined gaseous or liquid medium and which shall notimpede or obstruct the flow thereof. It is a still further object ofthis invention to provide an electrocaloric type flow meter inconnection with the measuring of the flow of gasoline, oil, coolingliquids and other flowing media in airplanes and which is accuratewithin a Wide range of temperatures and supply voltages. It is anotherprime object of this invention to provide a flow meter capable ofmeasuring the rate of flow of a flowing medium as well as the totalquantity of flow thereof. Other objects and advantages of my inventionwill appear from the description thereof hereinafter following.

The nature of the flow meter of the invention and its functioning areillustrated in the accompanying drawings, forming part hereof, in which:

Figure 1 represents a schematic illustration of one embodiment of theflow meter of the invention,

Figure 2 represents a schematic illustration of a modification of theembodiment of the flow meter of the invention shown in Figure 1,

Figure 3 represents a schematic illustration of another modification ofthe embodiment of the flow meter of the invention shown in Figure 1,

Figure 3A represents a modification of the heater circuit of theinvention,

Figure 4 represents a schematic illustration of still anothermodification of the embodiment of the flow meter of the invention shownin Figure 1,

Figure 4A represents an alternate location of the electrical connectionof the temperature compensation coil of the invention,

Figure 5 represents a schematic illustration of my invention as operableon A. C. current, and

Figure 6 represents a diagram of part of the circuit of the embodimentof Figure 5.

The flow meter of the invention involves the use of a heat exchanger inwhich a heating or cooling means constitutes one element, and theflowing medium constitutes the other element, heat being transferred toor withdrawn from the flowing medium and the temperature increase ordecrease occurring in the flowing medium being measured with respect toa known reference temperature of the flowing medium before passingthrough the heat exchanger. The flow meter of the invention alsoprovides means for readily transforming and calibrating such temperatureincrease or decrease in terms of rate of flow of the flowing medium orin terms of quantity of flowing medium, and further includes means foradjusting or compensating for certain errors which might otherwiseeffect the accuracy of the functioning of the device of the invention.The flow meter of the invention may be operated by D. C. current as wellas A. C. current and may be utilized either for the measuring of therate of flow of a flowing liquid or gas or of the total quantity of flowthereof in any given period of time.

In Figure l I have shown a conduit or pipe 1 through which the liquid orgaseous medium flows. This conduit or pipe line 1 contains a pipesection 2 of material of high thermal conductivity, i. e. e. g. metal ofhigh specific heat conductivity and low specific heat, such as copper,silver, aluminum, etc., spaced and heat insulated from the adjacentsections of the line by spacers or sections 3 and 4, which areadvantageously formed of material of low thermal conductivity such asrubber, Bakelite, synthetic rubber, e. g. neoprene, ceramic material, orthe like. Heat is transferred to the flowing medium through this heatconductive section 2 by means of a heater coil 5 wound on the outside ofa portion of the section 2. This heater coil 5 may be formed of anysuitable resistance element, e. g. a wire or ribbon of such metals oralloys as Nichrome, manganin, constantan, etc. A resistance thermometeroconsisting of a coil of thermo-responsive wire or ribbon is wound on theoutside of section 2. The heater coilprecedes the thermometer coil 6 inthe direction of the flow of the medium involved. Both coils may. beprotected against ambient temperature eflects by means of a tubularhousing 7, including end covers 8 and 9, the latter being in intimatethermal contact with the conduit 1. Wound on the conduit or pipe 1, at apoint before the flowing medium reaches the heat exchange section 2, isa reference resistance thermometer 10. The resistance thermometers 6 and10 may be of the usual type involving coils of wire or ribbonconstructed of a material the temperature coefficient of electricalresistance of which is high and reproducible, e. g. platinum or alloysof precious metal or nickel. A temperature compensating coil 50, asfully explained hereinafter is also mounted on pipe 1 (see thedescription of Figure 4). The windings of the resistance coils are woundin intimate thermal contact with the conduit. If insulated resistancewire is used the windings may be in direct contact with the surface ofthe metal conduit; otherwise a further thin coating of insulatinglacquer should be interposed between the metal 'con duit and the barewire.

1 The operation of the device of Figure 1 is more fully explainedhereinafter in connection with the description of Figure4'.

The difference in temperature between that of the flowing medium beforereaching the heat exchanger and that of the flowing medium after leavingthe heat exchanger is a function of the specific heat of the flowingmedium, of the quantity of heat transferred in the exchange, and of therate of flow of the flowing medium. If heat energy is introduced into amedium flowing within a conduit and its'temperature measured before andafter the heat exchange, the differential temperature At between the tworeadings will vary with the rate of flow. It will be small for high flowrates and large for low flow rates and if in such case the quantity ofheat transferred to the flowing medium is taken as Q, in terms of gramcalories per second; the specific heat of the flowing medium is taken asc; the quantity of flowing medium, in terms of grams per second, istaken as M; and the temperature rise, in terms of degrees C., is takenas At, the relationship of the various factors may be expressed in aformula as follows:

and if the wattage input W to a heater coil is proportional to Q theequation becomes:

At= (C.)

W=cMAt 2) and Consequently as long as the specific heat of the flowingmedium does not vary and the quantity of heat transferred is keptconstant, i. e. are known and constant, the temperature differential isinversely proportional to the rate of flow to be measured, so long asthe heat transferred is carried away by the flowing medium by storing itand conveying it to places of different temperature. Likewise, if thetemperature increment and the specific heat of the flowing medium areconstant the quantity of heat transferred is directly proportional tothe rate of flow of the flowing medium. 7

It will be understood that instead of measuring the temperature rise inthe flowing medium caused by transfer of heat thereto I may measure thetemperature drop caused by withdrawal of a given amount of heat from theflowing medium. In this case the heat exchanger would 4 embody anexternal cooling element instead of an external heater, e. g. in theform of a refrigerant.

The principle utilized in the flow meter of the invention involves,therefore, as one aspect thereof the measurement of the temperaturechange,occurring in the flowing medium after passing through a heatexchanger with the quantity of heat transferred being kept constant andthe change being inversely proportional to the rate of flow, or themeasurement or the amount of heat transfer with the temperature changebeing kept constant and the quantity of heat transfer being directlyproportional to the rate of flow.

Referring further to Figure 1, the resistance thermometers 6 and 10 areconnected to a Wheatstone bridge,

shown schematically in Figure 1 as including fixed resistances 20 and 21and a measuring instrument 22 in the diagonal ofthe bridge adapted tomeasure the temperature differential between such thermometers. If thetemperature gradient between the thermometers 6 and 10 is kept constantand the wattage input to the heating coil 5 is varied by a means 53, thewattage input to such heating coil 5'will be proportional to the rate offlow in the line, and a watt meter 25 connected into the circuit of theheater coil 5 can be calibrated to read the rate of flow. Or, thewattage input to the heater coil 5 may be kept constant, in which casethe temperature differential between the thermometers 6 and 10 will beinversely proportional to the rate of flow.

All component parts entering into the measurement of the rate of flow ofthe flowing medium are located outside the conduit containing theflowing medium and there are no movable parts inserted into the flowingmedium itself. There is, therefore, no obstruction which might stop orimpede or in any way interfere with the free flow of the liquid or othermedium involved. The construction does not embody any bushingsor'bearings through which any part of the flowing medium might leak out.The circuit elements employed for carrying the electrical current areseparated from the flowing medium by the walls of the conduit, thusavoiding any possibility of ignition or explosion of the medium. Thereare no obnoxious or toxic fumes which might otherwise escape from theconduit or pipe line. The embodiment of Figure 1 illustrates theapplication of my invention to a straight conduit.

Figure 2 of the drawings illustrates the application of the flow meterof the invention, wherein a bypass arrangementmay be used, if desired,with a view to requiring limited quantities of power input for the heatexchanger and yet producing asufficiently high temperature gradient toassure dependable accurate results.

In Figure 2 I provide a bypass conduit, whereby a portion only of theflowing medium is transmitted through the heat exchanger and is thussubjected to the measuring process, which said portion .is in a fixedrelation to the total quantity of the flowing medium. The bypass conduitmay be inserted into the flow conduit 1 of Figure las a unit and in suchcase consists of a normal flow conduit section 11 and the bypass conduitsection 12 This device may be connected to the regular line 1 by meansof fittings 13 and 14. The stream of the flowing Figure 1 which iselectrically arranged as shown in Figure 1.-

Iheflow through the bypasssection 12 is caused by the pressure dropbetween the inlet and outlet of the bypass section 12. In the embodimentillustrated in Figure 2 this pressure drop is relatively smallunless thedistance between the inlet and the outlet points is made sufficientlylong which can be done in most but not all installations.

In 'Figure 3 I have illustrated another embodiment of my inventionsimilar to that of Figure 2 but adapted to maintain a sufiiciently largeand well defined pressure drop between the inlet and the outlet of thebypass section or line while yet keeping the total length of the deviceas short as possible. In this case the main conduit 1, or an insertedsection thereof, is built as a Venturi tube and the bypass section lineis provided with its inlet opening at a point ahead of the taper of theVenturi tube, e. g. at 28, and its outlet opening at the narrowest partor the throat of the Venturi tube, at 29. The bypass line 30 is similarto the bypass conduit 12 of Figure 2 and a section thereof intermediateits length is provided with the housing 7 containing the heat transferelements shown in Figure 1 which is electrically arranged as shown inFigure l.

t In any of the embodiments of the invention there may be connected, asschematically shown in Figure 3A, in series with the heater coil, alimit switch 37, the contacts of which are normally closed, therebypermitting current to flow to the heater coil 5. The contacts of thislimit switch can be caused to open when an unwanted limiting conditionof the fluid adjacent the heater coil occurs, e. g. an undesirably hightemperature or low flow rate of the fluid. In the event the criterion isthe temperature of the fluid then the switch 37 may be any of thethermostatic switches which are per se well known in the art. Such athermostatic switch is adjusted so that ifthe temperature of the pipesection in the neighborhood of the heater exceeds a predetermined value,the contacts of the switch 37 are opened automatically. Such a switch ispreferably of the bimetallic thermostatic type and may consist of astrip of bimetallic metal wound or coiled around the conduit. Thisarrangement is especially desirable where the flowing medum beingmeasured has a high vapor pressure at room temperature or is otherwisecapable of evaporating easily. For instance, in the case where gasolineis the flowing medium, the temperature at very low flow rates, or atzero flow rate, might otherwise become so high as the result of theoperation of the heater that a vapor lock might form which wouldobstruct the flow of the gasoline through the bypass conduit or thatbubbles might develop in the gasoline which would make the flow of thegasoline irregular. Such a thermostatic switch not only interrupts orcontrols the current to the heater in the case of such excessive heatdevelopment, but is also adapted to protect the measuring instrumentmeasuring the temperature dilference be tween the resistancethermometers against overload at small flow rates. In the event thecriterionis the flow .rate of the fluid then the switch 37 may be any ofthe flow actuated switches which are per se well known in the art, inwhich the switch remains closed only so long as the flow rate in thepipe exceeds a predetermined flow rate, but when the flow rate dropsbelow this value the contacts of the switch are opened automatically.This arrangement is also desirable in the case where gasoline .is theflowing medium since, as indicated above, at very of the flowing mediumthrough the bypass line 30, as a result of the change in the velocity ofthe flowing medium between the widest point of the tube, indicated at28, and the narrowest part of the tube or throat 29. This pressure dropcan be controlled by the diameter of the entrance to the Venturi tube.

The range of flow covered by the flow meter can be changed by changingthe wattage input to the heater.

Figure 4 illustrates another embodiment of the flow meter of theinvention in which a high pressure drop in the flow conduit is obtainedby inserted orifices or flow nozzles instead of by means of a Venturitube arrangement. The main conduit 1 is again tapped by a bypass conduit41 similar to that shown in Figures 2 and 3. The bypass conduit isprovided with the heater 5 and the differential between the temperaturein the main conduit and the temperature in the bypass line is measuredby thermometers it and 6, respectively. In this embodiment I provideorifices or flow nozzles in the main conduit and in the bypass line. Theorifice 45 in the main conduit is provided intermediate the bypass inletand outlet, and preferably close to the bypass outlet, as indicated. Asimilar orifice or flow nozzle 46 may be provided in the bypass line 41.The orifices or nozzles are preferably inserted in flange connections 45and 48 respectively, so that they are easily replaceable, e. g. forcalibration purposes, and so that they are exchangeable for orifices ofdifferent dimensions to change the sensitivity of the flow meter bychanging the ratio of the bypass flow to the main flow. By usingspecially designed tapered nozzles or orifices such as those described]by W. Koennecke in Archiv fiir Technisches Messen V, 1242-2, January1939, a pressure drop can be obtained between the inlet and the outletwhich is practically constant within a wide range of Reynolds numbers.As the Reynolds number characterizing the condition of a flowing medium,varies considerably when the viscosity of the fluid varies with thetemperature thereof, I may use a nozzle or orifice of such specialdesign, as e. g. the quarter-circle type described in the Archiv, whichproduces a constant pressure drop for widely varying conditions of flowsuch for instance as may be caused by changes in the temperature of theflowing medium. By suitable combination of such orifices or nozzles inthe main conduit 1 on the one hand and in the bypass line 41 on theother hand, it is possible to make the ratio of flow in the two linessufficiently constant for a wide range of temperatures, thus avoidingthe introduction of occurrence of temperature errors. However, othernozzles or ordinary orifices may of course be used.

Varying fluid temperatures may effect the accuracy of flow measurementsand my invention encompasses a method of temperature compensation.Normally the thermometer arms are made of material having a hightemperature coeificient of electrical resistance, such as nickel wire,and their resistance changes equally if the temperature of the flowingmedium or of the ambient air changes, and the equilibrium of theWheatstone bridge is thus maintained. Ordinarily, however, sizeabletemperature error is left, e. g. due to a change in the flow ratiobetween the flow in the main conduit and the flow in the bypass conduitor the change in the specific heat of the flowing medium with varyingtemperature, etc. For example, referring to the Equation 2 above, it isseen that the quantity of heat W transferred to the fluid is directlyproportional to the flow rate M as long as the specific heat of thefluid remains constant. However, for most liquids the specific heatvaries somewhat over a wide range of temperatures. Furthermore we haveto consider the effect of temperature on the viscosity of the fluid,which in creases for gases and decreases for liquids with increasedtemperature. This aifects the character of the flow and the mechanism ofthe heat transfer between the coils and the fluid. It is readilyrecognizable that the local velocity of a fluid within a conduit is byno means uniform and that since the velocity distribution is governed bythe Reynolds spasm" number which is inversely proportional to viscosityit is, therefore, a function of fluid temperature. It is obvious,therefore, that the heat transfer from the wall to the fluid is affectedby the character of the flow in the neighborhood of the wall or byboundary layer conditions, that is, by the temperature of the fluid.Hence, when the fluid temperature varies, considerable error may beintroduced into the instrument. These problems were non-existent inknown types of calorimeter flowmeters where the resistance elements havebeen mounted within the flow conduits. Moreover, those types offlowmeters which do mount the temperature sensitive. elements woundaround the outside of the flow conduit, such as that described in theSwedish patent to J. P. Lutz, published on June 2, 1942, recognize thatsuch an instrument as previously known required as a condition foraccuracy in measurement that the fluids whose flow is to be measuredexhibit constant'values of specific heat, density, composition,temperature and a constant degree of turbulent flow. My invention,therefore, enables such calorimetric devices as shown by Lutz to beapplicable in accurately measuring the flow of any fluid by removing allerrors due to temperature fluctuations. I

Such errors and other errors due to unknown transient conditions whichdepend on temperature variations can be compensated for by making one ofthe remaining arms of the bridge (e. g. arm 20, which normally would bemade of manganin or similar wire the resistance of which does not varywith temperature) slightly sensitive to temperature as by adding theretoa small section of a wire coil 50 of nickel or other temperaturesensitive material whose resistance increases with increase intemperature, wound on the conduit 1 so as to be responsive to andsubject to its temperature, whereby the Wheatstone bridge indicatesunbalance only as the result of a change in the rate of flow of theflowing medium.

Another method of compensating for any residual temperature error is toshunt the heater coil (consisting of wire the resistance of which doesnot vary with temperature) with a bypass coil 50a similar to coil 50described above so that with raising temperature the shunt resistancewill increase and a larger share of the wattage will go into the heatercoil as shown in Figure 4A. In this instance a thermoconstant resistance18 is connected in series with the source of E. M. F. and the heater.This arrangement may be substituted for the bridge connected coil in anyof the embodiments of the invention.

As shown in Figures 1 and 4, the resistance thermometers 6 and 10 areconnected to a Wheatstone bridge supplied with power from the battery51. The signal output of the bridge is normally zero but if the flowrate in the main flow conduit 1 changes, a signal will be produced bythe bridge and the measuring instrument 22 in the bridge diagonal willshow a deflection from zero in one direction or the other, dependingupon whether the rate of flow in the conduit 1 is increasing ordecreasing. Such a-measuring instrument is per se well known in the artand may be a'sirnple nullindicator which is read by the operator whothen manually adjusts the wattage input to-the heater by operating therheostat 53 which is connected in series with the heater coil 15 and thepower supply 51. The bridge signal is thus restored to zero byincreasing or decreasing the wattage input to the heater coil. Themeasuring instrument 22 may also be a controlling galvanometer or a nullrelay which is equipped with high and low contacts and is also per sewell known in the art. Such an automatic controller or controllinggalvanometer is used to control the power input to a reversible speedmotor which drives the rheostat, i. e. rheostat 53 in Figure 4, todecrease or increase the wattageinput to the heater, in a manner whichis per se well known in the electromechanical controls art, e. g. thearrangement for the controlling galvanometer can be analogous to thatshown schematically in Figure for use'with an electronic controller.

8 7 Frequently it is desirable to measure not only the rate offlow butalso the integrated flow, i. e. the total quantity of the medium flowingthrough the conduit over any specified period of time. Referring to theFormula 3 previously stated, it will be noted that by integrating bothsides of the equation over time the equation becomes:

if 2' denotes the time.

The left side of Equation 4 represents the totalized flow which is seento be proportional to the watt hour consumption of the heater. The watthour consumption of the heater can be measured with a watt hour meter inthe heater circuit, which can be calibrated to read the integrated flowdirectly in pounds or gallons. One example of useful application of theflow meter of the invention as a measuring device for the totalintegrated flow is the measurement'of the flow and consumption ofgasoline, e. g. in aircraft engines, so that the pilot can check notonly the rate of consumption of gasoline at any time but also the totalconsumption up to any given time, whereby the safety, speed and economyof air travel can be greatly increased. a

In Figure 5 I have illustrated a flow meter according to the inventiondesigned for and operating specifically on alternating current. Themodification illustrated in Figure 5 also describes automatic control ofthe wattage I input to the heater by means of an electronic controllerin place of the sensitive relay or galvanometer type controllerpreviously referred to. a

In this embodiment I have not shown the coils mounted in any specialrelationship on the conduit since the coils 5, 6, 10 and perform thesame functions and may be mounted on any of the conduit arrangementsshown'in the previous illustrations. The resistance thermometers 6 and10 form two arms of a Wheatstone bridge, which is completed by two otherfixed resistor arms 20 and 21 and the temperature error compensatingmeans 50 is shown as connected into the arm 20. The Wheatstone bridge isconnected through a series resistance 76 of thermo-constant material toa source 77 of alternating current of any conveniently availablefrequency and voltage. The output diagonal of the Wheatstone bridge isconnected to the primary side of a transformer 78 whose secondary sideis connected to an electronic voltage amplifier'79. Such amplifier 79may consist for instance of a number of resistance coupled triodesconnected in series and designed to amplify the relatively weak signalfrom the Wheatstone bridge to a voltage sufficiently high to drive thegrids of a power amplifier 80 which forms the second stage of theelectronic amplifier. Any conventional type of electronic poweramplifier can be used, a preferred example comprising triodes operatingin parallel and resulting in an output current sufiiciently large toenergize one coil of a two phase reversible induction motor 81 thesecond phase of which is connected to the A. C. power source 77 by meansof a capacitor 82. The power amplifier 80 is designed at the same timeto act as a phase discriminator, shifting the phase of its outputcurrent 180 if the voltage signal from the Wheatstone bridge goesthrough zero and shifts its phase 180, which will occur every time thebridge goes through its balance condition from an overbalanced to anunderbalanced condition or vice versa. Figure 6 illustrates such phasediscriminating circuit for the power amplifier 80 of Figure 5. Thevoltage amplifier 79 is applied across the condenser 83 and the resistor84 both of which are connected in series. The plates of the two triodes85 and 86 respectively, or of sets of triodes connected in parallel areconnected to the secondary winding of the transformer 87 the primary ofwhich is connected to the A. C. power source 77. The center tap 92 ofthe secondary winding of the transformer 87 is connected to one coil 88of the two phase induction motor'81, the second phase of which, 89, isconnected to the A. C. current source 77 by means of the condenser 82.The cathodes of the two triodes or sets of triodes 85 and 86,respectively, are connected in parallel and through resistor 90 to theother end of phase 88 of the motor 81. Condenser 91 is connected inparallel to this phase 88. The circuit described hasphase-discriminating characteristics, so that if the phase of theincoming voltage signal applied across the combination of. condenser 83and resistor 84 shifts 180 the phase of the current through the coil 88of the motor 81 will simultaneously shift 180. Since, however, the phaseof the current in coil 89 of the motor 81 is fixed in relation to thephase of the A. C. power supply 77 the end result of .a phase shiftincoil 88 will be a change in the direction of rotation of the motor 81.

As shown in Figure the motor 81 is mechanically coupled through a geartrain to a movable contact of a variable rheostat 92 which is in serieswith the heater coil 5 and the A. C. power source 77. Also in serieswith the heater coil 5 are an A. C. watt meter 93 and, if desired, an A.C. watt hour meter 94. The current through the heater coil 5 and meters93 and 94, respectively, is controlled by the position of the movablecontact of rheostat 92. If, therefore, the Wheatstone bridge becomesunbalanced due to a change in flow conditions in the conduit an A. C.signal will be produced in the output diagonal of the bridge, amplifiedin voltage amplifier 79 and power amplifier 80, and will be transmittedfinally to the motor 81 which, therefore, will begin to rotate andadjust the resistance of the rheostat 92 until balance is restored andthe signal from the Wheatstone bridge disappears. If flow conditionschange in the opposite direction an A. C. signal will likewise beproduced in the output diagonal of the Wheatstone bridge, the phase ofwhich in this case is shifted 180, and a current of the same shift inphase in coil 88 of the motor 81 will be produced, causing the motor 81to rotate in the opposite direction. The electronic controller will thuskeep the bridge in balance under any and all flow conditions in thetransmitting conduit. The amount of wattage required to maintain thebridge in balance is, as explained previously, indicative of the flowrate.

The electronic controller illustrated in Figures 5 and 6 possesses muchhigher sensitivity than the contact making relay or the galvanometertype controller, so that with the use of such electronic controller itis possible to still further reduce the size and thermal mass of thecomponents of the flow meter, thus reducing to a negligible amount anytime lag caused by such components. The resultant reduction in time lagassists greatly in the elimination of hunting and oscillation of theautomatic control.

This application is a division of my prior filed application SerialNumber 671,179, filed on May 21, 1946, now abandoned.

It will be seen, therefore, that I have provided a highly efficient flowmeter for the measuring of the rate of flow or of the quantity of flowof flowing media in a confined conduit, operable on D. C. or A. C.current, without any interference with the free flow of the flowingmedium and which eliminates errors due to fluctuations in thetemperature of the fluid.

What I claim is:

1. A flow meter for measuring the flow of a confined flowing mediumcomprising conduit means, heating means externally contacting a portionof said conduit means to enable transmission of heat energy through saidconduit means to said flowing medium, a first temperature responsivemeans externally contacting said conduit means and so located as to beresponsive to the temperature of the flowing medium prior to thetransmission of heat thereto, a second temperature responsive meansexternally contacting said conduit means and so located as to beresponsive to the temperature of the flowing medium after thetransmission of heat thereto, a bridge having a pair of fixed resistanceratio arms, the other arms con- 10 sisting of said first and secondtemperature responsive means and constituting the thermometer arms ofthe bridge, an electrical circuit comprising a first and a secondportion, said first portion consisting of said thermometer arms, saidsecond portion including said ratio arms and the heating means, meansarranged in the diagonal of said bridge for sensing any unbalancetherein as produced by the said temperature responsive means, and acompensating element externally wound in intimate thermal contact withsaid conduit means on a portion thereof containing flowing mediumunheated by said heating means so. as to be responsive to temperaturefluctuations of the flowing medium prior to the transmission of heatthereto, the compensating element and temperature responsive means beingin the same thermal transfer relationship with the flowing medium, saidelement being electrically connected into the second portion of saidcircuit whereby any sensing errors due to fluctuations in thetemperature of the flowing medium are compensated for by a shifting ofthe bridge balance and a signal is produced only as the result of achange in the rate of flow.

2. The flow meter of claim 1 wherein the thermometer arms of the bridgeare resistors made of a material having a high positive temperaturecoefiicient of electrical resistance, said ratio arms constituting apair of resistances of a material the resistance of which does not varywith temperature, and said compensating element is electricallyconnected in series with one of the fixed resistance ratio arms.

3. The flow meter of claim 1 wherein the heating means consists of acoil of wire the resistance of which does not vary with temperature andthe compensating element is electrically arranged to shunt the heatercoil and is of a material the resistance of which varies withtemperature, whereby upon the said temperature fluctuations occurring inthe flowing medium the shunting element automatically operates to varythe Wattage input to said heater coil to reduce to zero the signal ofsaid bridge so that it will only produce a signal as the result of achange in the rate of flow of the flowing medium.

4. The flow meter of claim 1 wherein the conduit means consists of amain conduit and a smaller branched bypass conduit interconnected withsaid main conduit at least said heating means and said secondtemperature responsive means being located on the branched bypassconduit.

5. The flow meter of claim 4 wherein the main flow conduit is providedwith a flow restricting means so located in relation to the exitjunction of said bypass conduit as to produce a head in said mainconduit and the first temperature responsive means is located on themain flow conduit prior to the flow restricting means.

6. The flow meter of claim 1 wherein said electrical circuit includes avariable power supply means connected to said heater, said sensing meansin the diagonal of said bridge being a controller means including anelec tric motor interconnected to vary the power output of said variablepower supply means, said controller means being responsive to signalsproduced by said bridge where" by said motor operates in connection withsaid power supply means to vary the current input into said heatingmeans to restore to zero the signal of the bridge, and power consumptionrecording and indicating means connected in series with said heatingmeans to indicate and record the amount of power required to maintainthe bridge balance.

7. The flow meter of claim 1, including a limiting switch means which isnormally in closed position, electrically connected to be in series withthe heating means, and responsive to a predetermined limiting conditionof the fluid adjacent to the heater means to open the circuit to theheating means and discontinue the flow of power thereto when saidpredetermined limiting condition is reached.

8. The flow meter of claim 1, including a normally closed thermostaticswitch means which is electrically connected to betin series with theheating means, and,

responsive to a predetermined temperature level of the fluid in saidconduit adjacent to the heating means, whereby when said temperaturelevel is reached the switch means operates to open the circuit to theheating means and discontinue the flow of power thereto.

9. The flow meter of claim 1, including a normally closed flowresponsive switch means which is electrically connected to be in serieswith the heating means, and responsive to a predetermined flow ratelevel of the fluid in said conduit adjacent to the heating means,whereby when said flow rate levelis reached the switch means op 7References Cited in the file of this patent UNITED STATES PATENTS1,222,492 Thomas Apr. 10, 1917 I 1,261,086 Wilson et a1. Apr. 2, 19182,067,645 Pinkerton Ian. 12, 1937 10 2,176,502 Kurth Oct. 17, 1939 IFOREIGN PATENTS Sweden June 2, 1942

